Crystallization and Dissolution in Aqueous Solution: A Bond-Valence Approach

Abstract

In many groups of minerals, structural diversity occurs by polymerization of a small number of clusters (or fundamental building blocks). Where these minerals crystallize from aqueous or hydrothermal solutions, the fundamental building blocks occur as aqueous species in solution, and it seems reasonable to conclude that crystallization of these minerals occurs by condensation of these clusters in solution. The variation in Lewis acidity of these clusters is a function of the pH of the aqueous solution in which they occur, in accord with the different structures crystallizing from similar aqueous solutions at different pH. Strongly bonded polyhedron chains (equivalent to periodic bond-chains) control the morphology of crystals. Anions at the surface of a mineral (i.e., exposed to an ambient aqueous solution) are called terminations, and the residual valence at a termination controls its reactivity (i.e., is the driving force for reaction with the aqueous solution). The residual valence of a polyhedron chain controls the growth or dissolution rate at the crystal face associated with that chain and may be calculated as the net residual valence of the terminations per repeat of the polyhedron chain. Edges involving polyhedron chains with low normalized residual valence will grow slowly, whereas edges involving polyhedron chains with high normalized residual valence will grow rapidly, and the relative morphology of crystals will be controlled by the relative magnitudes of the residual valence of polyhedron chains parallel to specific faces. The observed morphology of selected uranyl-oxide hydroxyl-hydrate and borate minerals is in reasonable accord with this approach.